For a turbocharger, it is often desirable to control the flow of exhaust gas into the turbine to improve the efficiency or operational range. In a variable nozzle turbine (“VNT”), variable geometry members (e.g., vanes) are employed to control the exhaust gas flow. Typically, multiple pivoting vanes annularly positioned around the turbine inlet and commonly controlled to alter the throat area of the passages between the vanes is a commonly used design.
A typical VNT includes an actuator capable of actuating the vanes through a range of flow positions that extends from a hard-stop closed position (a position where the vanes have closed off the exhaust flow to a point at which they are physically stopped) to a hard-stop open position (a position where the vanes have opened up the exhaust flow to a point at which they are physically stopped). A sensor device can be used to detect the actuation of the actuator, and thereby the position of the vanes.
During normal operation, turbocharger vanes are actuated through a range of positions, the most closed of which is a minimum-flow position that corresponds to a predetermined, fixed mass flow value. Due to tolerances and clearances in the variable geometry system (including the actuation system), there is variability of the vane position and the related actuation position for any given turbocharger when set for a given mass flow. Consequently, the actuation system of each turbocharger must be calibrated during assembly to ensure that it will direct the actuation of the vanes to the appropriate (minimum-flow) position to achieve the fixed mass flow value.
As the turbocharger ages, component wear occurs in the actuation system of the variable nozzle mechanism. As this wear builds up in actuation system components such as rods and gears that mechanically link the actuator to the VNT vanes, the wear creates a slowly increasing offset between the intended vane position and the actual vane position achieved under aerodynamic loading. Due to this drift, a previously identified minimum-flow actuator actuation position may no longer be accurate, and as a result, the mass flow may fail to achieve the desired mass flow value when the actuator is directed to actuate the vanes to that minimum-flow position. As a result, the turbocharger and related engine operate less efficiently, and emissions increase. These changes can manifest as a noticeable reduction in vehicle performance and changes in transient behavior.